The Eye and the Retina

The human eye

The eye is made up of three layers, which enclose the optically clear Aqueous humour, Lens, and Vitreous body.

The outermost coat consists of the Cornea and the Sclera.

The middle coat contains the main blood supply to the eye and consists of the Choroid, the Ciliary body, and the Iris (from the back forward).

The innermost layer is the Retina (yellow on the image below), lying on the Choroid and receiving most of its nourishment from the vessels within the Choroid. The remainder of its nourishment is received from the retinal vessels that lie on its surface and are visible in an ophthalmoscope or on fundus photographies.

The human retina

The retina is the third and inner coat of the eye which is a light-sensitive layer of tissue. The optics of the eye create an image of the visual world on the retina (through the cornea and lens), which serves much the same function as the film in a camera.

Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses. These are sent to various visual centres of the brain through the fibres of the optic nerve. This entire process is what makes it possible to see.

The term "Neural retina" typically refers to three layers of neural cells (photo receptor cells, bipolar cells, and ganglion cells) within the retina, while the "entire retina" refers to these three layers plus a layer of pigmented epithelial cells.

Fundus images / fundus photos

A fundus photograph is used to screen for and diagnose multiple eye diseases including diabetic retinopathy, AMD and glaucoma. The image is captured using a specialised camera (called a fundus camera). The goal is to spot disease-related changes in the retina in order to treat them early and save vision/prevent blindness.

Below is a 45° degree (angle of view) image of healthy a human retina (the central part of the retina). This fundus photograph shows the blood vessels; Veins are darker and slightly wider than corresponding arteries. The optic disc is on the right side (annotated in blue), and the macula is near the centre (marked in green). The central part of the macula is called the fovea (marked in yellow).

For reference, here is the same image without annotations:

The optic disc / optic nerve head (ONH)

The optic disc or optic nerve head is the point of exit for ganglion cell axons leaving the eye. In layman's terms, this is where all the signal cables are bundled and routed to the brain. Because there are no rods or cones overlying the optic disc, it corresponds to a small blind spot in each eye.

The optic disc can be categorised into two central parts, which are important to know when screening for papilledema (abnormal accumulation of fluid) and glaucoma; The Cup and the Disc represented in green and blue below.